The present invention relates to free piston internal combustion engines, and, more particularly, to piston assemblies used in free piston internal combustion engines.
Free piston internal combustion engines include one or more pistons which are reciprocally disposed within corresponding combustion cylinders. However, the pistons are not interconnected with each other through the use of a crankshaft. Rather, each piston is typically rigidly connected with a plunger shaft which is used to provide some type of work output. For example, the plunger shaft may be used to provide electrical power output by inducing an electrical current, or fluid power output such as pneumatic or hydraulic power output. In a free piston engine with a hydraulic output, the plunger is used to pump hydraulic fluid which can be used for a particular application. Typically, the housing which defines the combustion cylinder also defines a hydraulic cylinder in which the plunger is disposed and an intermediate compression cylinder between the combustion cylinder and the hydraulic cylinder. The combustion cylinder has the largest inside diameter; the compression cylinder has an inside diameter which is smaller than the combustion cylinder; and the hydraulic cylinder has an inside diameter which is still yet smaller than the compression cylinder. A compression head which is attached to and carried by the plunger shaft at a location between the piston head and plunger head has an outside diameter which is just slightly smaller than the inside diameter of the compression cylinder. A high pressure hydraulic accumulator which is fluidly connected with the hydraulic cylinder is pressurized through the reciprocating movement of the plunger during operation of the free piston engine. An additional hydraulic accumulator is selectively interconnected with the area in the compression cylinder to exert a relatively high axial pressure against the compression head and thereby move the piston head toward the top dead center (TDC) position.
Pistons used in free piston internal combustion engines typically include a piston head which is entirely constructed from a metallic material such as aluminum or steel. Metals such as aluminum and steel have a relatively high coefficient of thermal expansion. Thus, during operation of the free piston engine, the metallic piston head expands considerably in the radial direction toward the inside surface of the combustion cylinder. Each piston head used in the free piston engine is thus formed with an outside diameter which provides a considerable radial clearance with the inside surface of the combustion cylinder to accommodate the relatively large radial expansion during operation. To prevent blow-by of combustion products past the piston head during operation, the outside peripheral surface of the piston head is formed with one or more piston ring grooves which receive corresponding piston rings therein. The piston rings allow for radial thermal expansion and contraction of the piston head, while at the same time effectively preventing blow-by of combustion products past the piston head.
A problem with using conventional piston and cylinder arrangements is that suitable fluid cooling channels must be provided within the combustion cylinder to effect the proper cooling of the combustion cylinder and piston head. These cooling fluid channels increase the size and complexity of the engine. Moreover, the sliding interface between the piston and cylinder may not provide adequate cooling of the piston.
An example of a piston used in a free piston internal combustion engine is disclosed in U.S. Pat. No. 6,105,541 (Berlinger), assigned to the assignee of the present invention.
The present invention is directed to overcoming one or more of the problems as set forth above.
In one aspect of the invention, a free piston internal combustion engine is provided with a combustion cylinder and a piston assembly. The piston assembly includes a piston and a plunger shaft. The piston is reciprocally movable within the combustion cylinder. The piston includes a crown, a skirt extending from the crown and having a piston ring groove therein, a hub attached to the plunger shaft, and at least one support block. Each support block is positioned adjacent to the crown and extends from the hub toward the skirt. Each support block, skirt and piston ring groove defines a cut-out therebetween.